Electrochemical Reduction Of CO2 Research Articles

Probing the Activity and Selectivity of Nickel Carboxylethylphosphonate 2D-MOF for the Electrochemical Conversion of CO2 to Syngas

The increasing concentration of CO2 in the atmosphere is alarming for modern society and the reduction of CO2 into valuable products would be a unique solution.1 Metal–organic frameworks (MOFs), constructed from organic linkers interconnected with metal (oxide) nodes, with high porosity and large surface area, have become an emerging class of electrocatalysts for reduction of CO2.2,3 Herein, we report the synthesis and characterization of a two-dimensional nickel metal-organic framework (2D Ni-MOF) with 3-phosphopropionic acid and 4,4-bypyridine as ligands and Ni2+ as the node (figure 1). The electrocatalytic activity of this material is explored for the electrochemical CO2 reduction reaction (CO2RR). An onset potential for the electrochemical reduction CO2 activity of -0.51 V vs RHE was observed which is 70 mV more positive than the HER in Ar-saturated electrolyte suggesting an efficient CO2 reduction. This catalyst showed a high current density of 33.5 mA cm-2 at 1.01 V vs RHE, exhibiting superior performance to most of the MOFs tested so far for CO2 reduction (figure 2a and b). Long term electrolysis at -0.8 V (vs RHE) yielded a current density of ∼10 mA cm-2 with high selectivity towards the formation of CO and H2 with an average production ratio of 30:70%, respectively (figure 2c and d). This new Ni-MOF catalyst retained its crystallinity and morphology after electrolysis as confirmed by powder XRD, HRTEM, FT-IR and Raman spectroscopy, indicating promising stability of the catalyst.Density functional theory (DFT) was employed to calculate adsorption-free energy of the reaction intermediates to identify the active catalytic sites responsible for CO2 reduction in our 2D Ni-MOF structure. Findings show that the catalyst more readily reduces H* to H2 (with a limiting potential of 0.48 eV) than CO2 to COOH* (limiting potential of 0.78 eV). The reaction mechanism upon which this calculation was based begins with the removal of the H2O molecule bound to the Ni (II) site, followed by CO2 adsorption and the two proton-coupled electron transfer steps yielded COOH* and then CO & H2O. The H2O is more strongly adsorbed to the Ni (II) site than CO which closes the mechanistic cycle. Reference 1. M. J. B. Kabeyi and O. A. Olanrewaju, Frontiers in Energy Research, 2022, 9.2. B.-X. Dong, S.-L. Qian, F.-Y. Bu, Y.-C. Wu, L.-G. Feng, Y.-L. Teng, W.-L. Liu and Z.-W. Li, ACS Applied Energy Materials, 2018, 1, 4662-4669.3. J.-X. Wu, S.-Z. Hou, X.-D. Zhang, M. Xu, H.-F. Yang, P.-S. Cao and Z.-Y. Gu, Chemical Science, 2019, 10, 2199-2205. Figure 1

Toward Sustainable Olefins: Electroreduction of CO2 to Multi-Carbon Olefins Using Trimetallic Cu-Rich Catalysts

The sustainable conversion of carbon monoxide (CO2) into value-added chemicals via electrochemical methods, powered by renewable electricity, is considered as a promising approach to meet the global energy demand and close the anthropogenic carbon cycle. However, this technology is in the early stage of development, particularly in the production of multi-carbon (C2+) hydrocarbons due to well-studied thermodynamic and kinetic barriers. To overcome these challenges, developing a catalyst with high activity and selectivity is crucial to minimize energy input and maximize the effectiveness of C2+ products formation.In this work, we will present our recent advancement on developing non-precious tri-metallic catalysts with copper-rich surface synthesized by our developed atomically controllable and environmentally friendly synthesis method. The developed nanostructured tri-metallic electrocatalysts by this method show remarkable selectivity and activity in the electrochemical conversion of CO2 to olefins. In particular, our electrochemical results show an overall C2+ olefine Faradaic efficiency (FE%) of 69% more specifically 58% ethylene and 11% propylene at a current density of -112 mA/cm² under applied potential of -2.8 V in an anion exchange membrane (AEM) electrolyzer. To gain a better understanding of the catalytic performance and find a structural-performance relationships for the developed materials, we performed several physicochemical and electrochemical characterization experiments, including X-ray diffraction (XRD), scanning transmission electron microscopy (STEM), and X-ray photoelectron spectroscopy (XPS). We will discuss morphology and atomic structure changes of the developed materials under realistic conditions, with a specific focus on understanding how different trimetallic interfaces influence selectivity and activity in the electrochemical CO2 reduction reaction (eCO2RR).

Modeling Diurnal and Annual Ethylene Generation from Solar-Driven Electrochemical CO2 Reduction Devices

Integrated solar fuels devices for CO2 reduction (CO2R) are a promising technology class towards achieving net-negative carbon emissions. Designing integrated CO2R solar fuels devices requires careful co-design of electrochemical and photovoltaic components as well as consideration of the diurnal and seasonal effects of solar irradiance, temperature, and other meteorological factors expected for ‘on-sun’ deployment. Using a photovoltaic-electrochemical (PV-EC) platform, we developed a temperature and potential-dependent diurnal and annual model using experimental CO2R performance of Cu-based electrocatalysts, local meteorological data from the National Solar Radiation Database (NSRD), and modeled performance of commercial c-Si PVs. We simulated diurnal product outputs with and without the effects of ambient temperature to determine gaseous product temperature sensitivity. From these outputs, we observed seasonal variation in gaseous product generation, with up to two-fold increases in ethylene productivity between the Winter and Summer, analyzed the consequences of dynamic cloud coverage, and identified periods where device cooling/heating mechanisms could be implemented to maximize ethylene generation. Finally, we modeled the annual ethylene generation for a scaled 1 MW solar farm at three different locations (Beijing, CN; Sydney, AUS; Barstow, CA) to determine the consequences of local meteorological climates on PV-EC CO2R product output, recording a maximum ethylene output of 18.5 tonne/yr at Barstow. Overall, this model presents a critical tool for streamlining the translation of experimental solar-driven electrochemical research to real-world implementation.

Electrocatalytic Upgrading of CO2 to Light Olefins in Protonic Ceramic Electrochemical Cells

Light olefins (ethylene, propylene, butylene) are primary building blocks for chemical manufacturing of large volume products including organic chemicals, plastics, and even sustainable aviation fuels. Currently, the dominant commercial process for olefin production is steam cracking of naphtha or ethane, which is highly energy intensive and accounts for a large fraction of global CO2 emissions. To decarbonize olefin production processes, electrochemical CO2-to-olefin conversion coupled with renewable or low-emission energy sources including electricity and heat offers an attractive alternative. Previously, extensive research efforts have been devoted to low temperature electrochemical CO2 reduction in aqueous media, i.e., occurring below 100 °C. This approach, however, still faces enormous scientific and technological challenges in improving CO2 utilization, energy efficiency, and product selectivity. In this work, we have developed a new electrochemical process for direct conversion of CO2 to ethylene at intermediate temperatures (350-500°C) in an electrochemical membrane reactor. This process is based on high-performance protonic ceramic electrochemical cells (PCECs) that integrate CO2 hydrogenation reaction to produce olefins in the cathode and concurrent hydrogen generation reaction in the anode to provide hydrogen for CO2 conversion. Highly active and selective catalysts for CO2-to-olefin conversion have been identified and integrated into the PCECs. Both the catalytic and electrochemical performances of the integrated reactor system are evaluated and optimized. In contrast to the use of molecular H2 in conventional thermal catalytic pathways, the active hydrogen in PCECs could be sourced from different feedstocks including H2O, light alkanes, and ammonia, etc. This work also provides valuable insights into the selection and design of efficient catalysts for electrochemical CO2 conversion at elevated temperatures.

Advancements in Electrocatalysts for Low-Temperature CO2 Electrolysis: Progress and Insights from the ECO2Fuel Project

The Electrochemical CO2 Reduction Reaction (CO2RR) represents a cutting-edge approach for converting CO2 into valuable chemicals and fuels, marking a significant stride in energy storage through chemical bonds. This innovative method ingeniously merges the well-established water splitting reaction with Fischer-Tropsch synthesis, traditionally dependent on high temperatures and pressures. However, uniquely, CO2RR facilitates this process under ambient conditions, combining hydrogenation with water splitting at room temperature and normal atmospheric pressure. In this context, the ECO2Fuel project, an initiative under the European Green Deal, aims to develop, implement, and validate the first-ever 1MW low-temperature direct electrochemical CO2 conversion system, poised to produce economically viable and sustainable e-fuels and chemicals.Despite its transformative potential, the ECO2Fuel project acknowledges and addresses several pivotal challenges, including enhancing catalyst performance and selectivity, electrode longevity, mass transport optimization, averting electrolyte flooding, and mitigating the hydrogen evolution reaction.This presentation will predominantly concentrate on refining the gas diffusion electrode at the cathode, for large-scale fuel and alcohol production via CO2RR. This electrode is composed of a gas diffusion layer, ensuring efficient CO2 transport; a catalyst layer where the reduction transpires; and a top adhesion layer to prevent catalyst delamination. These components collectively influence the electrode’s selectivity, durability, and stability.Our discourse will delve into the intricate aspects of the catalyst layer and porous transport layer design, along with electrode manufacturing techniques, demonstrating their critical roles in achieving high faradaic efficiencies for carbonaceous fuel production. Furthermore, we will explore the nuanced operational modalities of the CO2 electrolyzer cell, focusing on strategies to curtail the parasitic side reaction of hydrogen evolution, thereby enhancing the system's overall efficiency and productivity.This project has received funding from the European Union’s Horizon 2020 research and Innovation program under grant agreement no. 202037389.

Effect of the EMIM Cation on the CO2 Reduction Reaction on Gold

The electrochemical CO2 reduction reaction (CO2RR) is emerging as a promising method to produce carbon neutral chemical feedstocks. Additives or co-catalysts in the electrolyte can lower the activation barrier for CO2RR, yielding faster conversion rates at lower voltages. Recently, electrocatalytic CO2 reduction mediated by ionic liquids (IL) has been reported but with conflicting accounts to whether the ILs benefit CO2RR or adversely, the competing hydrogen evolution reaction (HER).We report the effect of added IL EMIM-Cl, specifically EMIM+, in aqueous solutions on the electrochemical CO2RR and competing HER on gold. We show the CO2 reduction current increases by ~1.5x with the highest amount of added EMIM+ relative to the pure electrolyte. Differential pulse voltammetry reveals an additional large change in the reduction current that only appears with both CO2 and EMIM+ present. Lastly, we use advanced electrochemical analysis methods to determine the reaction kinetics and preliminary evidence shows that the standard heterogeneous rate constant increases for the reaction at large overpotentials while the HER is suppressed at low overpotentials with EMIM+ present.Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.

Unlocking the Potential for Methanol Synthesis via Electrochemical CO2 Reduction Using CoPc-Based Molecular Catalysts.

The electrochemical CO2 reduction reaction (CO2RR) to produce methanol (CH3OH) is an attractive yet challenging approach due to a lack of selective electrocatalysts. An immobilized cobalt phthalocyanine (CoPc) molecular catalyst has emerged as a promising electrocatalyst for CH3OH synthesis, demonstrating decent activity and selectivity through a CO2-CO-CH3OH cascade reaction. However, CoPc's performance is limited by its weak binding strength toward the CO intermediate. Recent advancements in molecular modification aimed at enhancing CO intermediate binding have shown great promise in improving CO2-to-CH3OH performance. In this Perspective, we discuss the competitive binding mechanism between CO2 and CO that hinders CH3OH formation and summarize effective molecular modification strategies that can enhance both the binding of the CO intermediate and the conversion of the CO2-to-CH3OH activity. Finally, we offer future perspectives on optimization strategies to inspire further research efforts to fully unlock the potential for methanol synthesis via the CO2RR using molecular catalysts.

Electrochemical CO2Reduction Reaction: Comprehensive Strategic Approaches to Catalyst Design for Selective Liquid Products Formation.

The escalating concern regarding the release of CO2 into the atmosphere poses a significant threat to the contemporary efforts in mitigating climate change. Amidst a multitude of strategies for curtailing CO2 emissions, the electrochemical CO2 reduction presents a promising avenue for transforming CO2 molecules into a diverse array of valuable gaseous and liquid products, such as CO, CH3OH, CH4, HCO2H, C2H4, C2H5OH, CH3CO2H, 1-C3H7OH and others. The mechanistic investigations of gaseous products (e.g. CO, CH4, C2H4, C2H6 and others) broadly covered in the literature. There is a noticeable gap in the literature when it comes to a comprehensive summary exclusively dedicated to coherent roadmap for the designing principles for a selective catalyst all possible liquid products (such as CH3OH, C2H5OH, 1-C3H7OH, 2-C3H7OH, 1-C4H9OH, as well as other C3-C4 products like methylglyoxal and 2,3-furandiol, in addition to HCO2H, AcOH, oxalic acid and others), selectively converted by CO2 reduction. This entails a meticulous analysis to justify these approaches and a thorough exploration of the correlation between materials and their electrocatalytic properties. Furthermore, these insightful discussions illuminate the future prospects for practical applications, a facet not exhaustively examined in prior reviews.

Temperature promotes selectivity during electrochemical CO2 reduction on NiO:SnO2 nanofibers.

Electrolyzers operate over a range of temperatures; hence, it is crucial to design electrocatalysts that do not compromise the product distribution unless temperature can promote selectivity. This work reports a synthetic approach based on electrospinning to produce NiO:SnO2 nanofibers (NFs) for selectively reducing CO2 to formate above room temperature. The NFs comprise compact but disjoined NiO and SnO2 nanocrystals identified with STEM. The results are attributed to the segregation of NiO and SnO2 confirmed with XRD. The NFs are evaluated for the CO2 reduction reaction (CO2RR) over various temperatures (25, 30, 35, and 40 °C). The highest faradaic efficiencies to formate (FEHCOO- ) are reached by NiO:SnO2 NFs containing 50% of NiO and 50% SnO2 (NiOSnO50NF), and 25% of NiO and 75% SnO2 (NiOSnO75NF), at an electroreduction temperature of 40 °C. At 40 °C, product distribution is assessed with in situ differential electrochemical mass spectrometry (DEMS), recognizing methane and other species, like formate, hydrogen, and carbon monoxide, identified in an electrochemical flow cell. XPS and EELS unveiled the FEHCOO- variations due to a synergistic effect between Ni and Sn. DFT-based calculations reveal the superior thermodynamic stability of Ni-containing SnO2 systems towards CO2RR over the pure oxide systems. Furthermore, computational surface Pourbaix diagrams showed that the presence of Ni as a surface dopant increases the reduction of the SnO2 surface and enables the production of formate. Our results highlight the synergy between NiO and SnO2, which can promote the electroreduction of CO2 at temperatures above room temperature.

Tuning the Inter-Metal Interaction between Ni and Fe Atoms in Dual-Atom Catalysts to Boost CO2 Electroreduction.

Dual-atom catalysts (DACs) are promising for applications in electrochemical CO2 reduction due to the enhanced flexibility of the catalytic sites and the synergistic effect between dual atoms. However, precisely controlling the atomic distance and identifying the dual-atom configuration of DACs to optimize the catalytic performance remains a challenge. Here, the Ni and Fe atomic pairs were constructed on nitrogen-doped carbon support in three different configurations: NiFe-isolate, NiFe-N bridge, and NiFe bonding. It was found that the NiFe-N bridge catalyst with NiN4 and FeN4 sharing two N atoms exhibited superior CO2 reduction activity and promising stability when compared to the NiFe-isolate and NiFe-bonding catalysts. A series of characterizations and density functional theory calculations suggested that the N-bridged NiFe sites with an appropriate distance between Ni and Fe atoms can exert a more pronounced synergy. It not only regulated the suitable adsorption strength for the *COOH intermediate but also promoted the desorption of *CO, thus accelerating the CO2 electroreduction to CO. This work provides an important implication for the enhancement of catalysis by the tailoring of the coordination structure of DACs, with the identification of distance effect between neighboring dual atoms.

Enhanced CO2 electroreduction to C2+ production on asymmetric Zn-O-Cu sites via tuning of *CO intermediate adsorption

The electrochemical CO2 reduction reaction conducted presents a promising strategy to facilitate the artificial carbon cycle. Unfortunately, the efficiency of eCO2RR-to-C2+ remains below the level required for large-scale implementation due to complex multi-electron transfer and sluggish carbon-carbon coupling. Herein, we constructed asymmetric Zn-O-Cu sites on 2.12 %Zn/CuOx, which achieving a maximum C2+ product FE of 78.77 ± 1.90 % and a high current density of 408.3 mA cm−2. Experimental and theoretical studies reveal that the O-bridged asymmetric Zn-O-Cu sites exhibit enhanced electron transfer, which plays a pivotal role in improving the coverage of *CO and adjusting the adsorption strength of the *CO. The optimal adsorption capacity of the *CO on 2.12 %Zn/CuOx facilitated the subsequent hydrogenation reaction to enhance the conversion of *CO to *COH. Consequently, the asymmetric Zn-O-Cu sites proved to be more thermodynamically favorable for the asymmetric coupling between *CO and *COH, which is conducive to the production of C2+ products.

Breaking the scaling relations of effective CO2 electrochemical reduction in diatomic catalysts by adjusting the flow direction of intermediate structures.

The electrocatalytic carbon dioxide reduction reaction (CO2RR) is a promising approach to achieving a sustainable carbon cycle. Recently, diatomic catalysts (DACs) have demonstrated advantages in the CO2RR due to their complex and flexible active sites. However, our understanding of how DACs break the scaling relationship remains insufficient. Here, we investigate the CO2RR of 465 kinds of graphene-based DACs (M1M2-N6@Gra) formed from 30 metal atoms through high-throughput density functional theory (DFT) calculations. We find that the intermediates *COOH, *CO, and *CHO have multiple adsorption states, with 11 structural flow directions from *CO to *CHO. Four of these structural flow directions have catalysts that can break the linear scale relationship. Based on the adsorption energy relationship between *COOH, *CHO and *CO, we propose the concepts of linear scaling, moderate breaking, and severe deviation regions, leading to the establishment of new descriptors that identify 14 catalysts with potential superior performance. Among them, ZnRu-N6@Gra and CrNi-N6@Gra can reduce CO2 to CH4 at a low limiting potential. We also discovered that DACs have independent bidirectional electron transfer channels during the adsorption and activation of CO2, which can significantly improve the flexibility and efficiency of regulating the electronic structure. Furthermore, through machine learning (ML) analysis, we identify electronegativity, atomic number, and d electron count as key determinants of catalyst stability. This work provides new insights into the understanding of the DAC catalytic mechanism, as well as the design and screening of catalysts.

A high-performance watermelon skin ion-solvating membrane for electrochemical CO2 reduction

Ion-solvating membranes have been gaining increasing attention as core components of electrochemical energy conversion and storage devices. However, the development of ion-solvating membranes with low ion resistance and high ion selectivity still poses challenges. In order to propose an effective strategy for high-performance ion-solvating membranes, this study conducted a comprehensive investigation on watermelon skin membranes through a combination of experimental research and molecular dynamics simulation. The micropores and continuous hydrogen-bonding networks constructed by the synergistic effect of cellulose fiber and pectin enable the hypodermis of watermelon skin membranes to have a high ion conductivity of 282.3 mS cm−1 (room temperature, saturated with 1 M KOH). The negatively charged groups and hydroxyl groups on the microporous channels increase the formate penetration resistance of watermelon skin membranes in contrast to commercially available membranes, and this is crucial for CO2 electroreduction. Therefore, the confinement of proton donors and negatively charged groups within three-dimensional microporous polymers gives inspiration for the design of high-performance ion-solvating membranes.

Enhanced CO2 Adsorption and Conversion in Diethanolamine‐Cu Interfaces Achieving Stable Neutral Ethylene Electrosynthesis

AbstractMolecular modifications have shown tremendous potential in boosting the electrochemical CO2 reduction (CO2RR) to ethylene. However, the key mechanisms of modulation at the molecular level remain unclear, especially for the adsorption and activation of key intermediates (e.g., *CO2 and *CO). Here, report that a diethanolamine (DEA)‐modified Cu catalyst can reduce CO2 to ethylene with a faradaic efficiency of ≈50.5% with a partial current density of ≈155.7 mA cm−2 in the neutral conditions, which surpasses the Cu catalyst without molecular modification (≈28.5% and ≈95.6 mA cm−2). Density functional theory calculations demonstrate that DEA on the Cu surface boosts the adsorption and activation of CO2 and the following C–C coupling processes during the CO2RR‐to‐ethylene process. Molecular dynamics simulations suggest that the molecules distant from the Cu site have a CO2 enrichment effect. Operational stability achieved via the introduction of DEA molecules onto ketjen black, which then successively immobilized on the Cu nanoparticles and polytetrafluoroethylene electrodes to obtain a stable tripe‐phase boundary, realizing constant ethylene selectivity for 100 operating hours in a flow cell.

Direct Microenvironment Modulation of CO2 Electroreduction: Negatively Charged Ag Sites Going beyond Catalytic Surface Reactions.

Electrochemical reduction of CO2 is an important way to achieve carbon neutrality, and much effort has been devoted to the design of active sites. Apart from elevating the intrinsic activity, expanding the functionality of active sites may also boost catalytic performance. Here we designed "negatively charged Ag (nc-Ag)" active sites featuring both the intrinsic activity and the capability of regulating microenvironment, through modifying Ag nanoparticles with atomically dispersed Sn species. Different from conventional active sites (which only mediate the surface processes by bonding with the intermediates), the nc-Ag sites could also manipulate environmental species. Therefore, the sites could not only activate CO2, but also regulate interfacial H2O and CO2, as confirmed by operando spectroscopies. The catalyst delivers a high current density with a CO faradaic efficiency of 97 %. Our work here opens up new opportunities for the design of multifunctional electrocatalytic active sites.

Ionic Liquids Functionalized Copper Catalytic Systems for Electrocatalytic Carbon Dioxide Reduction

The extensive combustion of fossil fuels results in excessive release of carbon dioxide (CO2), causing a global environmental crisis. It is imperative to develop sustainable methods for converting CO2 into renewable energy sources. Electrochemical reduction of CO2 (CO2RR) offers the potential to generate valuable chemicals, including C1 products (e.g., carbon monoxide, methane, etc.) and C2+ products (e.g., ethene, ethanol, acetic acid, propyl alcohol, etc.). Copper‐based (Cu‐based) catalysts show promise for producing value‐added C2+ products, but they face challenges like low selectivity and stability. The catalytic performances of Cu‐based catalysts can be promoted through electronic structure adjustment, selective crystal face exposure, as well as molecular additive approaches. Ionic liquids (ILs), known for their strong CO2 adsorption capacity, adjustable hydrophobicity, and wide chemical window, hold significant promise for addressing the current challenges associated with Cu‐based catalysts. This review provides a comprehensive overview of the structural characterization and catalytic mechanisms of ILs used in Cu‐based CO2RR catalytic systems. Additionally, it offers suggestions for future research avenues regarding IL‐functionalized Cu catalysts.

Design of Cr-Based Molecular Electrocatalyst Systems for the CO2 Reduction Reaction.

ConspectusHuman influence on the climate system was recently summarized by the sixth Intergovernmental Panel on Climate Change (IPCC) Assessment Report, which noted that global surface temperatures have increased more rapidly in the last 50 years than in any other 50-year period in the last 2000 years. Elevated global surface temperatures have had detrimental impacts, including more frequent and intense extreme weather patterns like flooding, wildfires, and droughts. In order to limit greenhouse gas emissions, various climate change policies, like emissions trading schemes and carbon taxes, have been implemented in many countries. The most prevalent anthropogenic greenhouse gas emitted is carbon dioxide (CO2), which accounted for 80% of all U.S. greenhouse gas emissions in 2022. The reduction of CO2 through the use of homogeneous electrocatalysts generally follows a two-electron/two-proton pathway to produce either carbon monoxide (CO) with water (H2O) as a coproduct or formic acid (HCOOH). These reduced carbon species are relevant to industrial applications: the Fischer-Tropsch process uses CO and H2 to produce fuels and commodity chemicals, while HCOOH is an energy dense carrier for fuel cells and useful synthetic reagent. Electrochemically reducing CO2 to value-added products is a potential way to address its steadily increasing atmospheric concentrations while supplanting the use of nonrenewable petrochemical reserves through the generation of new carbon-based resources. The selective electrochemical reduction of CO2 (CO2RR) by homogeneous catalyst systems was initially achieved with late (and sometimes costly) transition metal active sites, leading the field to conclude that transition metal complexes based on metals earlier in the periodic table, like chromium (Cr), were nonprivileged for the CO2RR. However, metals early in the table have sufficient reducing power to mediate the CO2RR and therefore could be selective in the correct coordination environment. This Account describes our efforts to develop and optimize novel Cr-based CO2RR catalyst systems through redox-active ligand modification strategies and the use of redox mediators (RMs). RMs are redox-active molecules which can participate cocatalytically during an electrochemical reaction, transferring electrons─often accompanied by protons─to a catalytic active site. Through mechanistic and computational work, we have found that ligand-based redox activity is key to controlling the intrinsic selectivity of these Cr compounds for CO2 activation. Ligand-based redox activity is also essential for developing cocatalytic systems, since it enables through-space interactions with reduced RMs containing redox-active planar aromatic groups, allowing charge transfer to occur within the catalyst assembly. Following a summary of our work, we offer a perspective on the possibilities for future development of catalytic and cocatalytic systems with early transition metals for small molecule activation.

Steering the Site Distance of Atomic Cu-Cu Pairs by First-Shell Halogen Coordination Boosts CO2-to-C2 Selectivity.

Electrocatalytic reduction of CO2 into C2 products of high economic value provides a promising strategy to realize resourceful CO2 utilization. Rational design and construct dual sites to realize the CO protonation and C-C coupling to unravel their structure-performance correlation is of great significance in catalysing electrochemical CO2 reduction reactions. Herein, Cu-Cu dual sites with different site distance coordinated by halogen at the first-shell are constructed and shows a higher intramolecular electron redispersion and coordination symmetry configurations. The long-range Cu-Cu (Cu-I-Cu) dual sites show an enhanced Faraday efficiency of C2 products, up to 74.1 %, and excellent stability. In addition, the linear relationships that the long-range Cu-Cu dual sites are accelerated to C2H4 generation and short-range Cu-Cu (Cu-Cl-Cu) dual sites are beneficial for C2H5OH formation are disclosed. In situ electrochemical attenuated total reflection surface enhanced infrared absorption spectroscopy, in situ Raman and theoretical calculations manifest that long-range Cu-Cu dual sites can weaken reaction energy barriers of CO hydrogenation and C-C coupling, as well as accelerating deoxygenation of *CH2CHO. This study uncovers the exploitation of site-distance-dependent electrochemical properties to steer the CO2 reduction pathway, as well as a potential generic tactic to target C2 synthesis by constructing the desired Cu-Cu dual sites.

Selective electroreduction of CO2 to C2+ products on cobalt decorated copper catalysts

Cu-catalyzed electrochemical CO2 reduction reaction (CO2RR) to multi-carbon (C2+) products is often plagued by low selectivity because the adsorption energies of different reaction intermediates are in a linear scaling relationship. Development of Cu-based bimetallic catalysts has been considered as an attractive strategy to address this issue; however, conventional bimetallic catalysts often avoid metals with strong CO adsorption energies to prevent surface poisoning. Herein, we demonstrated that limiting the amount of Co in CuCo bimetallic catalysts can enhance C2+ product selectivity. Specifically, we synthesized a series of CuCox catalysts with trace amounts of Co (0.07-1.8 at%) decorated on the surface of Cu nanowires using a simple dip coating method. Our results revealed a volcano-shaped correlation between Co loading and C2+ selectivity, with the CuCo0.4% catalyst exhibiting a 2-fold increase in C2+ selectivity compared to the Cu nanowire sample. In situ Raman and Infrared spectroscopies suggested that an optimal amount of Co could stabilize the Cu oxide/hydroxide species under the CO2RR condition and promote the adsorption of CO, thus enhancing the C2+ selectivity. This work expands the potential for developing Cu-based bimetallic catalysts for CO2RR.

Diiron Complexes with Rigid and Conjugated S-to-S Bridges for Electrocatalytic Reduction of CO2.

Electroreduction of CO2 to value-added low-carbon chemicals is a promising way for carbon neutrality and CO2 utilization. It was found that the diiron complex [(μ-bdt)Fe2(CO)6] (bdt = benzene-1,2-dithiolate) has high catalytic activity for electrocatalytic CO2 reduction. To further study the effect of the S-to-S bridge on the catalytic performances of diiron complexes for electrochemical CO2 reduction, four diiron complexes 1-4 with different rigid and conjugated S-to-S bridges were either selected or designed. The electrocatalytic studies showed that under optimal conditions, 2 with a 2,3-naphthalenedithiolato bridge exhibited the lowest catalytic onset potential (Eonset = -1.75 V vs Fc+/0), while 4 with a diphenyl-1,2-vinylidene bridge displayed the highest catalytic activity (TOFmax = 295 s-1), which is 1.5 times that of [(μ-bdt)Fe2(CO)6]. The controlled potential electrolysis experiments of 4 in 0.1 M MeOH/MeCN at -2.35 V vs Fc+/0 gave a total faradaic yield close to 100%, with selectivities of 77%, 9%, and 14% for HCOOH, CO, and H2, respectively. The mechanism for CO2 reduction was studied using density functional theory, IR spectroelectrochemistry, and electrochemical methods. The results indicate that modifying the structure of the S-to-S bridge is an effective strategy to improve the catalytic performance of diiron complexes for electrocatalytic CO2 reduction.